China's low permeability oil and gas reserves are abundant and widely distributed, which has become an important target for exploration and development. Formation damage is prone to occur during drilling in low permeability reservoirs, and it is difficult to recover effectively in production. Therefore, to protect reservoirs, underbalanced drilling has become a common drilling technique in reservoir stratum of low permeability oil and gas fields. Because of the low support force of drilling fluid column pressure in the process of underbalanced drilling, if the formation strength is weak or subjected to strong in-situ stress, it is prone to instability and cause drilling complex problems. According to the reservoir core mechanics experimental results and geo-mechanical data of some low permeability oil and gas fields in western South China Sea, a feasibility evaluation model of underbalanced drilling in horizontal wells was established. Based on this model, underbalanced drilling feasibility and underbalanced limit pressure difference were analyzed for horizontal wells in Huangliu group of D13 gas field, Weizhou group of W11 oilfield and Zhuhai group of W10 gas field respectively. The calculation shows that underbalanced drilling can be used basically in the horizontal wells of above reservoirs, but the magnitude of underbalanced pressure difference is greatly affected by the in-situ stresses and formation strength. The analysis results can guide underbalanced drilling design in horizontal wells of low permeability reservoirs in different blocks.
Low-permeability oil and gas fields refer to oil and gas fields with low reservoir permeability and poor production capacity that need to be taken stimulation to maintain normal production. China's oil and gas industry standard "assessment methods for oil and gas reservoirs" (SY/T 6285–1997) stipulates that reservoirs with permeability below 50×10−3μm2 are low-permeability reservoirs (Zhao et al., 1998). Low-permeability oil and gas fields play an important role in the development of oil and gas in China. At present, over 50% of China's remaining proven reserves of oil and gas are low-permeability reservoirs (Hu, 2009; Tang et al., 2013). Compared with conventional oil and gas reservoirs, low-permeability oil and gas fields require the introduction of more advanced technologies in the exploration and development stage. Such as four-dimensional seismic exploration technology, reservoir detailed description and initial microstructure analysis technology, new crack identification and fracturing technologies, horizontal wells and multi-branch well technology, slim hole drilling technology, and underbalanced drilling technology, all of these are commonly used technologies in low-permeability oil and gas fields (Zhang et al., 2009; Wang et al., 2001).
